Plastic materials are very present in our environment. They are found everywhere and they contribute considerably to improve the well-being of humans. The reasons justifying their industrial and domestic uses are that plastics are light materials, resistant, chemically inert and economical to produce.
The extent and usefulness of plastic materials are incalculable. In spite of their benefits, they presently cause a serious problem of environmental pollution which can no longer be ignore. The recycling or the re-use of some plastic compounds only partially improves the situation, thus the interest to develop partially or totally biodegradable films.
At present, consumers require greater quality and longer shelf lives for their foodstuffs. At the same time, they demand a reduction in the quantity of packing materials used. Plastic represents one of the principal components used for the packaging of our consumer products. In 1992, according to Canada's Green Plan, packaging accounted for approximately 30% of solid waste of Canadian municipalities. As packaging and other plastics are basically resistant to the attacks of bacteria present in nature, it is no longer ecologically acceptable to dispose of our plastic wastes by burying them underground or discharging into the ocean. Alternatives to these means of disposal are thus recycling, incineration, composting and biological breakdown. Following the sensitizing of federal authorities to this problem, a national protocol on packaging was signed in April 1990 which aims at reducing wastes coming from packaging by 50% before the year 2000 (Supply and Services Canada).
Over the last few years, the general population has become more sensitized to the problems of pollution caused by plastics. This made it possible to develop starch based plastic films which are currently on the market (Pledge, 1990). The presence of starch contributes to facilitate the microbial attack via the enzymatic systems. The ultimate result sought by the addition of starch is the loss of structural integrity which translates into a loss in the molecular weight of plastic films.
At present, an active research continues in order to produce a biodegradable film which would not be detrimental for the environment and which would have the characteristics of plastic packaging. Then, one must look for a compound which would be initially biodegradable, which would resist thermal denaturation and which would show certain properties of plastic packaging.
The majority of films which are considered biodegradable are formed simply by the solubilization of degradable components in a suitable solvent. Until now, research was undertaken on films containing polysaccharides, proteins and lipids alone or in combination. Usually, the fool films are formed starting from polymers of high molecular weight in order to provide a sufficiently resistant and adherent matrix. The characteristics of the matrix will depend on the chemical structure of the polymer, on the presence of plasticizing agents and on the way in which the film is applied to the surface of products. The application of coating films to the surface of products is done by steeping, pulverization or extrusion. The principal properties sought for packaging films are resistance, malleability, impermeability to gases and moisture and the possibilities of conditioning.
A polymer is obtained by the polymerization of a monomer in order to form continuous and/or branched polymeric chains. In the case of plastic components, this polymerization is generally carried out either by heat (thermoplastic) as for the poly-iminocarbonate (Li and Kohn, 1989), or by a photosynthesis with ultraviolet radiation as for the formation of acrylic resin and methacrylic (Ciardelli et al., 1989). It is also possible to obtain polyurethane by gamma irradiation (Shintani and Nakamura, 1991). At more than 25 kGy, the effect of irradiation on polymer is limited mainly to intermolecular polymerization with little or no degradation. Proteinic polymerization can be also carried out by enzymatic processes. Indeed, collagen polymers (Richard-Blum and Ville, 1988), casein .alpha..sub.sl, (Moloki and Al, 1987) and fibrin (Kasai and Al, 1983) were obtained in this way.
A film's cohesive strength is linked to its polymeric and chemical structure, to the nature of solvent used, to the presence of plasticizing agents or additives and to the surrounding conditions during the formation of the film (Kester and Fennema, 1986).
A direct link exists between the cohesion of film and the length and the polarity of the chains of the polymer. A uniform distribution of the polar groupings along the polymeric chains increases cohesion by increasing the probability of ionic interactions and hydrogen bonding between the chains (Banker, 1966). Generally, when one increases the cohesive structure of a film, a reduction in its flexibility, its porosity and its permeability to gas, vapor and aqueous solutions is observed (Kester and Fennema, 1986). For example, the stability of the tertiary structure of a protein affects the formation of a film and the properties resulting from film, that is, molecular flexibility, contributes to the formation of cohesive films (Graham and Phillips, 1980).
The solvents used for the formation of edible films are limited to water, ethanol or a combination of both (Kester and Fennema, 1986). Tests with ammonia and acetic acid were carried out but a problem with odor limits their use. After its evaporation, ammonia has a persistent odor which is evident on the end product (Gontard et al., 1992).
The plasticizing agents are usually grouped in the polyol family such as glycerol, sucrose and others. They can be introduced in order to give flexibility to films thereby improving their mechanical properties. The plasticizing characteristic is obtained by reducing the intermolecular forces thus affecting the film structure and increasing the mobility of the polymer chains. However, the relaxation of the film structure reduces the ability of film to act as a barrier to the diffusion of several gases and vapors (Gennadios and Weller, 1990; Peyron, 1991).
Environmental conditions influence film cohesiveness. Excessive temperatures during drying result in rapid solvent evaporation. Theses conditions can prematurely immobilize the polymeric chains before they have gathered to form a continuous and coherent film (Banker, 1996). This can generate certain defects like micro-perforations or a non-uniform thickness which inevitably will increase the permeability of film.